There is an increasing demand to recycle foundry sands from moulds after casting. The demand is fuelled not only by the cost of virgin sand but also by the problems associated with the disposal of the used resin coated sand. In the past such material was readily disposed of in land fill sites but recently the authorities have become more environmentally conscious and in many regions there are strict regulations governing the disposal of such materials.
One known method of sand reclamation comprises attrition of the bonded sand to break up the agglomerates into individual particles. Whilst the attrition process may remove some resin from the sand particles by abrasion which will be removed with the fines, resin remains on the surface of sand particles and the re-bonding properties of the attrition reclaimed sand are inferior to the bonding properties of new sand. Generally, conventional attrition techniques allow re-use of up to 85% of the resin bonded sand, the remaining sand being dumped.
Known thermal techniques for reclaiming foundry sand after attrition comprise heating the sand in a fluidised bed to a sufficiently high temperature to remove the organic resin effectively and to ensure low emissions from the exhaust gas. However, it has been found that such a thermal reclamation process is not particularly successful with ester-cured bonded foundry sands because there is a tendency for the sand grains to agglomerate in the thermal reclaimer preventing efficient operation of the fluidised bed at temperatures high enough to remove the binder effectively and ensure low emissions. At low temperatures there is inefficient removal of the resin. Sand reclaimed by the known thermal techniques exhibits re-bonding properties inferior to new sand and comparable to sand reclaimed by attrition.
It is believed the problem of agglomeration in the thermal reclamation system is due to the presence of potassium in the resin binder system which is generally in the form of potassium hydroxide and associated ester salts. It is postulated that the potassium compounds decompose and/or melt during the thermal treatment which results in agglomeration of sand particles, the particles being bonded or attracted to each other to such an extent that the fluidising gas is unable to maintain an effective fluidised bed.
The potassium compounds could be removed by washing the foundry sand prior to thermal treatment. However, such washing would significantly increase the energy requirements to dry and thermally treat the washed sand that such a procedure would be uneconomic.